Date: Sun, 28 Mar 1999 17:18:37 +0200 From: Gerald Heinig <heinig@hdz-ima.rwth-aachen.de> To: danny@alpha.net.au, net@FreeBSD.ORG Subject: Re: OSI layering Query.. Correction :-( Message-ID: <36FE484D.5F569F9A@hdz-ima.rwth-aachen.de> References: <199903262358.JAA27392@sydney.alpha.net.au> <36FD9D67.4BFE39D@hdz-ima.rwth-aachen.de>
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Gerald Heinig wrote: > > > > > And what exactly are PDU? > > Protocol Data Units. The information packages/packets that get sent > up/down the protocol stacks during protocol operation/data > transmission/reception. For example, a TPDU (transport protocol data > unit) contains the TSAP address (transport service access point address > - corresponds loosely to the "port number" in IP) and the "payload" ie. > data you're trying to transmit. This gets sent down (if you're > transmitting) to the NSAP (network service access point - equivalent to > a network interface in IP) which adds its NSAP address (which > corresponds to the IP address in IP). This then gets sent down to the > link layer (layer 2) to the LSAP (link layer service access point, ie. > the network card) which adds its LSAP address (ie. MAC/hardware/ethernet > address) plus header/trailer/FCS (frame check sequence) and sends the > caboodle off onto the wire. Hi Danny, Sorry, the above is wrong, I just checked. T/N/LSAPs are just addresses, NOT interfaces, logical or physical. An NSAP would correspond to an IP address, an LSAP to a hardware address. A PDU is usually used to describe the data that two machines exchange at the same layer. For example, saying machine A sends an IP packet to machine B is the same as saying machine A sends a network PDU to machine B. Note this is a rather abstract view of things. You can also think of PDUs as the data that passes up and down the protocol stacks during transmission/reception. As the PDU passes further down the stack, each layer adds its SAP to it. So, a TPDU (transport PDU) has in its packet the payload (data you're trying to send) and the TSAP. The next layer down (network layer) adds its NSAP, the next layer its LSAP until the physical layer adds the FCS and sends the message off onto the wire. > > > > > What exactly is SAP?? > > Service Access Point. The place where you can access a service. A Correct. > telephone socket is the SAP for a connection-oriented voice data Wrong. Forget that. > transfer service. IP mail exchange has an (IP) transport service access > point address of 25 ie.it uses IP port 25 for mail exchange. Your > machine's network interface would be the NSAP, the interface's IP > address would correspond to its NSAP address. Note: *CORRESPOND*!!! Again, wrong. The machine's IP address would correspond to its NSAP. > There is such a thing as an NSAP address and it is *NOT* the same as an > IP address!!! we're talking analogies here! > > > > I am really confused help me. Looks like I was as well. :-) :-) It seems to be somewhat awkward to map OSI terminology directly to IP. Some books say OSI layers 3 *and* 4 are IP layer 3, some disagree. Sorry about any confusion I caused: I should have given the book another good read *before* posting, not after. :-( Gerald -- "Would you like to buy an encyclopaedia to help your child get to college?" "He doesn't need it. He takes the bus!" To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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